The present invention relates to a tire tread pattern and in particular to a pattern suitable for heavy vehicle radial ply tires such as those used on trucks and buses.
Modern vehicles run at high speeds often for very long distances, and radial ply truck tread patterns comprising two circumferentially extending main grooves providing in each of the tread edges a shoulder rib are prone to uneven wear at the very edges of the tread, i.e. the shoulders. This wear begins as a narrow step or `drop` in the shoulder but progresses across the tire as the tire wears so that the step becomes wider and deeper. The step however often is uneven around the tire and this results in a reduction in tire performance and also a reduction in the effective ground contact patch area. The appearance of the tire becomes progressively more untidy and this makes it particularly difficult for the user to accurately assess the state of wear of the tire which means that the tire is not used fully and economically. Such treads may have circumferential ribs or rows of blocks between the shoulder ribs.
Previous attempts to solve this uneven and unpredictable shoulder wear have included restricting the tread contact width by forming a step in the shoulder but this reduces the cost effectiveness of the tire with respect to the useful tread area for wear. Effectively, the carcass size/cost for the tire is made larger than should really be necessary for the tread width and thus the tire is more expensive.
Another attempt in the prior art uses a narrow, circumferentially-extending, continuous decoupling groove adjacent to the tread edge to form a narrow rib which itself has a reduced radius to the main tread radius, so that it has a step appearance. This design, although it gives good consistent tire appearance through the wear life of the tire, has an unnecessary reduction in the work efficiency of the tread zone as it effectively makes the narrow shoulder rib a non working rib i.e. not contributing to tread life.
Accordingly, it is an object of the present invention to provide a tire tread which avoids the uneven edge wear problem but retains from that edge region of the tire a useful contribution to tire performance and resulting tread life.